Continuous preparation of alkyl esters of (meth) acrylic acid

Abstract

In a process and an apparatus for the continuous preparation of alkyl esters of (meth)acrylic acid by reacting (meth)acrylic acid with alkanols having from 1 to 5 carbon atoms in a homogeneous, liquid, solvent-free phase at elevated temperature and in the presence of an acid esterification catalyst, in which the (meth)acrylic acid, the alkanol and the catalyst are fed to a reaction zone, the water formed is removed by rectification during a residence time as constituent of a mixture comprising alkanol in a rectification unit superposed on the reaction zone, the distillate thus obtained is separated into an organic phase comprising alkanol and an aqueous phase comprising water, the organic phase is returned to the rectification unit, the reaction mixture is discharged from the reaction zone and conveyed into a distillative separation zone comprising further rectification units and in the latter the alkyl (meth)acrylate formed is separated off.

We claim: 
     
       1. A process for the continuous preparation of alkyl esters of (meth)acrylic acid by reacting (meth)acrylic acid with alkanols having from 1 to 5 carbon atoms in a homogeneous, liquid, solvent-free phase at elevated temperature and in the presence of an acid esterification catalyst, in which the (meth)acrylic acid, the alkanol and the catalyst are fed to a reaction zone, the water formed is removed by rectification during a residence time as constituent of a mixture comprising alkanol in a rectification unit (III) superposed on the reaction zone, the distillate thus obtained is separated into an organic phase comprising alkanol and an aqueous phase comprising water, the organic phase is returned to the rectification unit (II), the reaction mixture is discharged from the reaction zone and conveyed into a distillative separation zone comprising further rectification units and in the latter the alkyl (meth)acrylate formed is separated off, wherein a) (meth)acrylic acid and an alkanol having from 1 to 5 carbon atoms are reacted in a molar ratio of from 1:0.75 to 1:2,   b) the organic phase formed in the rectification unit (III) is essentially completely returned to the rectification unit,   c) the aqueous phase formed in the rectification unit (III) is essentially removed from the system,   d) the reaction mixture discharged from the reaction zone is, with addition of water, fed to a further rectification unit (I) and in this is separated into a product (II) comprising the catalyst and the remaining (meth)acrylic acid and a product (I) comprising the alkyl ester of (meth)acrylic acid, remaining alkanol and water,   e) the product (II) formed in the rectification unit (I) is essentially completely returned to the reaction zone,   f) the product (I) from the rectification unit (I) is separated into an organic phase comprising the alkyl ester of (meth)acrylic acid and an aqueous phase and   g) the organic phase formed in the rectification unit (I) is fed to a further rectification unit (II) and in this the alkyl ester of (meth)acrylic acid is separated from the remaining alkanol and the remaining alkanol is returned to the reaction zone.   
     
     
       2. A process as claimed in claim 1, wherein the reaction zone comprises a cascade of at least two reaction regions connected in series, and the discharge stream of one reaction region forms a feed stream to a downstream reaction region, and wherein the temperature in the first reaction region is 70-105° C., and in the last region is 100-160° C., and wherein the reaction temperature rises along the cascade. 
     
     
       3. A process as claimed in claim 2, wherein the pressure in all reaction regions is from 100 mbar to atmospheric pressure, wherein the total residence time for the reactants in the reaction regions is from 0.25 to 15 hours, and wherein the residence time decreases in successive reaction regions. 
     
     
       4. A process as claimed in claim 1, wherein the rising vapors from the reaction regions are fed to a rectification unit (III) whose liquid runback is returned only to the first reaction region. 
     
     
       5. A process as claimed in claim 1, wherein the catalyst used comprises para-toluenesulfonic acid and/or an other organic sulfonic acid such as methanesulfonic acid, benzenesulfonic acid, dodecylbenzenesulfonic acid and/or sulfuric acid, wherein the content of catalytically active acid in the reaction zone, based on the reaction mixture present therein, is from 0.1 to 10% by weight, of para-toluenesulfonic acid and/or an amount equimolar thereto of another organic sulfonic acid and/or sulfuiric acid, wherein the content of catalytically active acid in the liquid phase of the rectification unit (I), based on the mixture present therein, is from 2.5 to 25% by weight of para-toluenesulfonic acid and/or an amount equimolar thereto of another organic sulfonic acid and/or sulfuric acid. 
     
     
       6. A process as claimed in claim 1, wherein both the (meth)acrylic acid and the catalyst are fed directly to the reaction zone, the alkanol to be esterfied, is n-butanol and is fed to the reaction zone via the rectification unit (III), wherein the rectification unit (III) is a rectification column, the reaction regions comprise reactors having convection vaporizers, and wherein the aqueous phase obtained at the top of the rectification unit (III) is completely discharged. 
     
     
       7. A process as claimed in claim 6, wherein the product (I) is separated into an organic phase comprising the n-butyl ester of (meth)acrylic acid and n-butanol and an aqueous phase, part of the aqueous phase is returned to the rectification unit (I), a liquid aqueous phase and a liquid organic phase are present in the rectification unit (I), the rectification unit (I) is a rectification column (I), the product mixture discharged from the reaction zone is fed to the lower part of the rectification column (I) and the water addition occurs in the upper part of the rectification column (I), and wherein part of the resulting organic phase comprising the n-butyl ester of (meth)acrylic acid and n-butanol is returned to the upper part of the rectification column (I). 
     
     
       8. A process as claimed in claim 6, wherein the product (II) formed in the rectification unit (I) and comprising the catalyst and the remaining (meth)acrylic acid is returned essentially completely to the reaction zone, preferably to the first reaction region, either directly and/or via the rectification unit (III), wherein part of the product (II) formed in the rectification unit (I) is discharged and fed to a distillation unit (IV) and in this is separated into a product (III) comprising n-butanol, (meth)acrylic acid and the n-butyl ester of (meth)acrylic acid and a product (IV) comprising the acid esterification catalyst and components having higher boiling points than the n-butyl ester of (meth)acrylic acid, and wherein the product (III) is returned to the rectification unit (I) and/or the reaction zone. 
     
     
       9. A process as claimed in claim 6, wherein the organic phase of the product (I) is fed to a rectification unit (II) and in this is separated into a) a product (V) comprising remaining n-butanol and components having lower boiling points than n-butyl (meth)acrylate, b) n-butyl (meth)acrylate and c) a product (VI) having a boiling point higher than n-butyl (meth)acrylate, the product (V) is returned to the reaction zone, via the rectification unit (III), the product (VI) is returned to the rectification unit (I), the rectification unit (II) is a rectification column (II), and wherein the product (V) is taken off in the upper part of the rectification column (II), the product (VI) is taken off from the bottom of the rectification column (II) and the n-butyl (meth)acrylate is taken off in vapor form while rising as a lateral branch stream in the lower part of the rectification column (II). 
     
     
       10. The process of claim 2, wherein said cascade has from 2 to 4 reaction regions. 
     
     
       11. The process of claim 2, wherein said reaction regions are separated from one another in space. 
     
     
       12. The process of claim 11, wherein the said cascade has from 2 to 4 reaction regions. 
     
     
       13. The process of claim 1, wherein the temperature in the first reaction region is from 80-130° C. and in the last reaction region is from 105-130° C. 
     
     
       14. The process of claim 3, wherein the pressure in all reaction regions is from 200 mbar to 700 mbar. 
     
     
       15. The process of claim 5, wherein said catalytically active acid is present in an amount of from 0.1 to 6% by weight, based on the reaction mixture present. 
     
     
       16. The process of claim 14, wherein the total residence time of the reactants in the reaction regions is from 1 to 7 hours. 
     
     
       17. The process of claim 15, wherein the total residence time of the reactants in the reaction regions is from 2 to 5 hours. 
     
     
       18. The process of claim 14, wherein the pressure in all reaction regions is the same.